The invention relates to a device for restoring lost fluid pressure, applicable in particular to riveting machines.
Conventionally, blind rivets of both plain and screw shanked types are clinched in simple, safe and practical manner using machines which when hand held and manually operated are often referred to as riveters, or rivet guns.
Most hand riveters are hydropneumatic in design, i.e. connected to a source of compressed air that is manually operated and actuates a hydraulic tool assembly with which the single rivets are rigidly associated.
The hydraulic tool assembly is biased normally by spring means into an at-rest configuration, which precedes the application of a rivet, and connected to a master cylinder filled with hydraulic oil and slidably and sealably accommodating a plunger set in motion by a pneumatic cylinder connected to the compressed air source.
Activation of the compressed air source forces the plunger into the chamber of the master cylinder, thereby displacing and directing the hydraulic oil into the tool assembly against the reaction of the relative spring means. The rush of oil into the hydraulic tool assembly has the effect of pulling the rivet in the axial direction, and subjecting it at the same time it to the necessary upsetting and clinching force.
Guns of the type in question are notably robust in construction, by reason of the high pressures that must be generated in producing the axial pulling force necessary to upset and clinch a rivet. The higher the pressures brought into play however, the greater the problems experienced with fluid tightness, whether due to leakage, capillarity or whatever cause; these affect moving parts above all, given that the integrity of dynamic seals is difficult to guarantee in the long term. Reduced efficiency of the sealing action in a hand riveter results in an undesirable reduction in the travel of the rivet shank.
More exactly, the hydraulic tool assembly comprises a rod to which the rivet shank is attached, and a piston reciprocating within a barrel of which the chamber connects with that of the master cylinder, the piston being biased by the spring means toward a normally at-rest position enabling attachment of the rivet shank; thus, any reduction in efficiency of the sealing action leads to a reduction in the quantity of hydraulic oil contained in the chambers of the master cylinder and tool cylinder.
Each time the pneumatic cylinder is activated, the plunger will effect the full stroke by virtue of its being connected directly to the source of compressed air, which is supplied steadily at unvarying pressure.
Given that the limit positions of the piston of the pneumatic cylinder and the piston of the hydraulic tool assembly are fixed, any egress of hydraulic oil gives place to a corresponding ingress of air; thus, when the pneumatic cylinder is activated, the plunger first compresses the air which has replaced the hydraulic oil lost by reason of the inadequate sealing action, whereupon the oil will be displaced from the master cylinder into the hydraulic tool assembly and the spring means duly compressed. Air being compressible however, the initial movement of the plunger produces no movement of the rod of the hydraulic tool assembly, and neither, obviously, any corresponding movement of the rivet, with the result that the distance traveled by the rivet is reduced.
Such a reduction in travel signifies a technically substandard application of the rivet, as the two heads will be separated ultimately by a distance greater than, or at best equal to the combined thickness of the elements to be fastened together. By contrast, the rivet must be subjected to the axial pulling force mentioned above precisely in order to upset the second head at a distance from the first effectively less than the thickness of the joined elements, to the added end of applying compressive force and thus ensuring a more stable connection.
Currently, any such fault is remedied by completely stripping down, overhauling and reassembling the gun, and thereafter, recharging the master cylinder and hydraulic tool assembly with oil to restore the prescribed operating pressure.
These operations can be scheduled to take place at regular periods and/or in the event of detecting a reduction in the operating stroke of the riveter, though in either instance, it must happen that a certain number of rivets will have bee fitted with insufficient clinching force.
Accordingly, the object of the present invention is to overcome the drawback referred to above, whereby the travel of a rivet shank is reduced during the clinching operation as the consequence of a less than fully efficient sealing action internally of the riveting machine.